c#键盘钩子抛出System.NullReferenceException

在讨论键盘挂钩时，这篇文章被抛出了很多。

所以我绑定了它所建议的类:

using System;
    using System.Collections.Generic;
    using System.Text;
    using System.Runtime.InteropServices;
    using System.Windows.Forms;
    namespace Utilities {
    /// <summary>
    /// A class that manages a global low level keyboard hook
    /// </summary>
    class globalKeyboardHook {
    #region Constant, Structure and Delegate Definitions
    /// <summary>
    /// defines the callback type for the hook
    /// </summary>
    public delegate int keyboardHookProc(int code, int wParam, ref keyboardHookStruct lParam);
    public struct keyboardHookStruct {
        public int vkCode;
        public int scanCode;
        public int flags;
        public int time;
        public int dwExtraInfo;
    }
    const int WH_KEYBOARD_LL = 13;
    const int WM_KEYDOWN = 0x100;
    const int WM_KEYUP = 0x101;
    const int WM_SYSKEYDOWN = 0x104;
    const int WM_SYSKEYUP = 0x105;
    #endregion
    #region Instance Variables
    /// <summary>
    /// The collections of keys to watch for
    /// </summary>
    public List<Keys> HookedKeys = new List<Keys>();
    /// <summary>
    /// Handle to the hook, need this to unhook and call the next hook
    /// </summary>
    IntPtr hhook = IntPtr.Zero;
    #endregion
    #region Events
    /// <summary>
    /// Occurs when one of the hooked keys is pressed
    /// </summary>
    public event KeyEventHandler KeyDown;
    /// <summary>
    /// Occurs when one of the hooked keys is released
    /// </summary>
    public event KeyEventHandler KeyUp;
    #endregion
    #region Constructors and Destructors
    /// <summary>
    /// Initializes a new instance of the <see cref="globalKeyboardHook"/> class and installs the keyboard hook.
    /// </summary>
    public globalKeyboardHook() {
        hook();
    }
    /// <summary>
    /// Releases unmanaged resources and performs other cleanup operations before the
    /// <see cref="globalKeyboardHook"/> is reclaimed by garbage collection and uninstalls the keyboard hook.
    /// </summary>
    ~globalKeyboardHook() {
        unhook();
    }
    #endregion
    #region Public Methods
    /// <summary>
    /// Installs the global hook
    /// </summary>
    public void hook() {
        IntPtr hInstance = LoadLibrary("User32");
        hhook = SetWindowsHookEx(WH_KEYBOARD_LL, hookProc, hInstance, 0);
    }
    /// <summary>
    /// Uninstalls the global hook
    /// </summary>
    public void unhook() {
        UnhookWindowsHookEx(hhook);
    }
    /// <summary>
    /// The callback for the keyboard hook
    /// </summary>
    /// <param name="code">The hook code, if it isn't >= 0, the function shouldn't do anyting</param>
    /// <param name="wParam">The event type</param>
    /// <param name="lParam">The keyhook event information</param>
    /// <returns></returns>
    public int hookProc(int code, int wParam, ref keyboardHookStruct lParam) {
        if (code >= 0) {
            Keys key = (Keys)lParam.vkCode;
            if (HookedKeys.Contains(key)) {
                KeyEventArgs kea = new KeyEventArgs(key);
                if ((wParam == WM_KEYDOWN || wParam == WM_SYSKEYDOWN) && (KeyDown != null)) {
                    KeyDown(this, kea) ;
                } else if ((wParam == WM_KEYUP || wParam == WM_SYSKEYUP) && (KeyUp != null)) {
                    KeyUp(this, kea);
                }
                if (kea.Handled)
                    return 1;
            }
        }
        return CallNextHookEx(hhook, code, wParam, ref lParam);
    }
    #endregion
    #region DLL imports
    /// <summary>
    /// Sets the windows hook, do the desired event, one of hInstance or threadId must be non-null
    /// </summary>
    /// <param name="idHook">The id of the event you want to hook</param>
    /// <param name="callback">The callback.</param>
    /// <param name="hInstance">The handle you want to attach the event to, can be null</param>
    /// <param name="threadId">The thread you want to attach the event to, can be null</param>
    /// <returns>a handle to the desired hook</returns>
    [DllImport("user32.dll")]
    static extern IntPtr SetWindowsHookEx(int idHook, keyboardHookProc callback, IntPtr hInstance, uint threadId);
    /// <summary>
    /// Unhooks the windows hook.
    /// </summary>
    /// <param name="hInstance">The hook handle that was returned from SetWindowsHookEx</param>
    /// <returns>True if successful, false otherwise</returns>
    [DllImport("user32.dll")]
    static extern bool UnhookWindowsHookEx(IntPtr hInstance);
    /// <summary>
    /// Calls the next hook.
    /// </summary>
    /// <param name="idHook">The hook id</param>
    /// <param name="nCode">The hook code</param>
    /// <param name="wParam">The wparam.</param>
    /// <param name="lParam">The lparam.</param>
    /// <returns></returns>
    [DllImport("user32.dll")]
    static extern int CallNextHookEx(IntPtr idHook, int nCode, int wParam, ref keyboardHookStruct lParam);
    /// <summary>
    /// Loads the library.
    /// </summary>
    /// <param name="lpFileName">Name of the library</param>
    /// <returns>A handle to the library</returns>
    [DllImport("kernel32.dll")]
    static extern IntPtr LoadLibrary(string lpFileName);
    #endregion
}
}

它可以工作一段时间，但使用电脑一段时间后，它会抛出一个系统。随机按下一个键的NullReferenceException。尤其是组合键。

是什么导致了这种情况，如何修复?

EDIT:这是我用来启动钩子的代码:

globalKeyboardHook globalKeyboardHook = new globalKeyboardHook();
private void Form1_Load(object sender, EventArgs e)
{
    globalKeyboardHook.KeyDown += gkh_KeyDown;
    globalKeyboardHook.KeyUp += gkh_KeyUp;
    globalKeyboardHook.hook();
}

这是我得到的完整错误:

类型为"System"的未处理异常。得到NullReferenceException"在System.Windows.Forms.dll中发生附加信息:对象引用未设置为对象的实例。

private static keyboardHookProc callback;
public void hook()
{
    callback = new keyboardHookProc(hookProc);
    var hhook = SetWindowsHookEx(1, callback, IntPtr.Zero, 0);
}